context: use atomic operation in ctx.Err

oos: darwin
goarch: arm64
pkg: context
cpu: Apple M1 Pro
               │ /tmp/bench.0.mac │          /tmp/bench.1.mac           │
               │      sec/op      │   sec/op     vs base                │
ErrOK-10             13.750n ± 1%   2.080n ± 0%  -84.87% (p=0.000 n=10)
ErrCanceled-10       13.530n ± 1%   3.248n ± 1%  -76.00% (p=0.000 n=10)
geomean               13.64n        2.599n       -80.94%

goos: linux
goarch: amd64
pkg: context
cpu: Intel(R) Xeon(R) CPU @ 2.30GHz
               │ /tmp/bench.0.linux │         /tmp/bench.1.linux          │
               │       sec/op       │   sec/op     vs base                │
ErrOK-16               21.435n ± 0%   4.243n ± 0%  -80.21% (p=0.000 n=10)
ErrCanceled-16         21.445n ± 0%   5.070n ± 0%  -76.36% (p=0.000 n=10)
geomean                 21.44n        4.638n       -78.37%

Fixes #72040

Change-Id: I3b337ab1934689d2da4134492ee7c5aac8f92845
Reviewed-on: https://go-review.googlesource.com/c/go/+/653795
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

src/context/benchmark_test.go

@@ -188,3 +188,23 @@ func BenchmarkDeepValueSameGoRoutine(b *testing.B) {
 		})
 	}
 }
+
+func BenchmarkErrOK(b *testing.B) {
+	ctx, cancel := WithCancel(Background())
+	defer cancel()
+	for b.Loop() {
+		if err := ctx.Err(); err != nil {
+			b.Fatalf("ctx.Err() = %v", err)
+		}
+	}
+}
+
+func BenchmarkErrCanceled(b *testing.B) {
+	ctx, cancel := WithCancel(Background())
+	cancel()
+	for b.Loop() {
+		if err := ctx.Err(); err == nil {
+			b.Fatalf("ctx.Err() = %v", err)
+		}
+	}
+}

src/context/context.go

@@ -428,7 +428,7 @@ type cancelCtx struct {
 	mu       sync.Mutex            // protects following fields
 	done     atomic.Value          // of chan struct{}, created lazily, closed by first cancel call
 	children map[canceler]struct{} // set to nil by the first cancel call
-	err      error                 // set to non-nil by the first cancel call
+	err      atomic.Value          // set to non-nil by the first cancel call
 	cause    error                 // set to non-nil by the first cancel call
 }
 
@@ -455,10 +455,11 @@ func (c *cancelCtx) Done() <-chan struct{} {
 }
 
 func (c *cancelCtx) Err() error {
-	c.mu.Lock()
+	// An atomic load is ~5x faster than a mutex, which can matter in tight loops.
-	err := c.err
+	if err := c.err.Load(); err != nil {
-	c.mu.Unlock()
+		return err.(error)
-	return err
+	}
+	return nil
 }
 
 // propagateCancel arranges for child to be canceled when parent is.
@@ -482,9 +483,9 @@ func (c *cancelCtx) propagateCancel(parent Context, child canceler) {
 	if p, ok := parentCancelCtx(parent); ok {
 		// parent is a *cancelCtx, or derives from one.
 		p.mu.Lock()
-		if p.err != nil {
+		if err := p.err.Load(); err != nil {
 			// parent has already been canceled
-			child.cancel(false, p.err, p.cause)
+			child.cancel(false, err.(error), p.cause)
 		} else {
 			if p.children == nil {
 				p.children = make(map[canceler]struct{})
@@ -545,11 +546,11 @@ func (c *cancelCtx) cancel(removeFromParent bool, err, cause error) {
 		cause = err
 	}
 	c.mu.Lock()
-	if c.err != nil {
+	if c.err.Load() != nil {
 		c.mu.Unlock()
 		return // already canceled
 	}
-	c.err = err
+	c.err.Store(err)
 	c.cause = cause
 	d, _ := c.done.Load().(chan struct{})
 	if d == nil {
@@ -639,7 +640,7 @@ func WithDeadlineCause(parent Context, d time.Time, cause error) (Context, Cance
 	}
 	c.mu.Lock()
 	defer c.mu.Unlock()
-	if c.err == nil {
+	if c.err.Load() == nil {
 		c.timer = time.AfterFunc(dur, func() {
 			c.cancel(true, DeadlineExceeded, cause)
 		})